The metallic tube in a hot finishing state is delivered to a cold working process, when the metallic tube does not satisfy requirements in quality, strength, or dimensional accuracy. Generally, examples of the cold working process include a cold drawing method in which a die and a plug or a mandrel bar are used and a cold rolling method in which a cold pilger mill is used.
In the cold rolling method with the cold pilger mill, diameter reducing rolling is performed to a hollow shell between a pair of rolls having a tapered groove die whose diameter is gradually reduced in a circumferential direction and a tapered mandrel bar whose diameter is also gradually reduced in a lengthwise direction. That is, the groove dies are made in the circumferences of the pair of rolls, and the groove dies have such shapes that the grooves become narrower as the rolls are rotated. The roll is repeatedly advanced and retreated along the tapered mandrel bar while rotated, whereby the rolling is performed to the hollow shell between the roll and the mandrel bar (for example, see “Iron and Steel Handbook third edition” vol. 3, (2) Steel Bar, Steel Tube, and Rolling Common Facilities, pp. 1183 to 1189).
FIG. 1 is a view showing a rolling principle of the cold pilger mill, FIG. 1A is an explanatory view showing a start point of a forward stroke, and FIG. 1B is an explanatory view showing a start point of a backward stroke. As shown in FIG. 1, in the cold pilger mill, a pair of rolls 2, which has a tapered groove die 3 whose diameter is gradually reduced from an engaging inlet side of the roll toward a finishing outlet side thereof, and a tapered mandrel bar 4, whose diameter is gradually reduced from an engaging inlet side of the tapered mandrel bar toward a finishing outlet side thereof, are used according to an outside diameter and a wall thickness (respectively, do and to in the figure) of a hollow shell 1 and an outside diameter and a wall thickness (respectively, t and d in the figure) of a rolled tube 5 of a product, and forward and backward strokes in which the wall thickness is decreased while the diameter of the hollow shell 1 is reduced are repeated.
At the start point of the forward stroke and the start point of the backward stroke in the reciprocating motion, a rotation angle of about 60° and feed ranging from about 5 to about 15 mm are intermittently imparted to the tube material (hollow shell 1), so that the rolling is repeatedly performed to a new portion.
The cold rolling with the cold pilger mill has an extremely high working ratio of the tube material, and about ten-time elongation can be performed. Advantageously, the cold rolling has a large effect of straightening an eccentricity of the wall thickness of the tube, a reducing process is not required, and the cold rolling has a high production yield. At the same time, the cold rolling with the cold pilger mill has a disadvantage of extremely low productivity compared with the cold drawing method. Therefore, the cold rolling with the cold pilger mill is mainly suitable to cold working of high grade tubes, such as a stainless steel tube and a high alloy steel tube, in which raw material cost and intermediate treatment cost are expensive. In a copper and copper alloy fabricated industry, high-efficiency production is realized by three-strand rolling, and the cold pilger mill becomes a core production process for copper and copper alloy fabricated products.